Theres a pretty reliable way to test: warm up for 10 mins or so with a few strides. Then with your hrm, start a 30min time trial. 5min slightly slower than the pace you think you can hold for 30min, 20 mins as hard as you can for the rest of the 30min without dropping HR, finnicky the last five as hard as you can. Average the last 20 minutes of the TT and put that into a calculator here on this site. I'm pretty sure its just +5%...